Applying Duct Mastic

Mastic Gives Ducts the Treatment

New home buyers often link comfort with central heating and cooling. In most houses, central systems use ductwork to distribute conditioned air. However, buyers don't know—and building contractors are just now learning—that the comfort of a ducted central system comes with an energy penalty. Recent research shows that ducts contribute 20 to 60 percent of the air leakage in houses.

Duct Mastic
To reduce air leakage, residential duct systems are starting to receive the same treatment as much larger systems in commercial buildings. The treatment is called duct mastic. Although the name implies adhesive, duct mastic doesn't hold ducts together. That's a job for screws, rivets and straps.

Mastic is really a flexible sealant. It varies in consistency from that of mashed potatoes to yogurt. Because it never fully hardens, mastic stretches as the duct (or other surface) expands and contracts. It withstands wetness. Some mastics contain tiny strands of fiberglass to increase strength.

Mastic comes in several packages: cartridges (like caulk), tubs and buckets. You can apply it with a caulking gun, brush, trowel or with your gloved hand. In the past, most mastics used a petroleum solvent base, but manufactures now offer many water-based products. These are safer to handle and easier to clean up.

Mesh Tape
For gaps greater than 1/4 in. and joints that may move, mastic should be reinforced with fiberglass mesh tape. This mesh is the same material used on drywall joints, but don't go to the drywall supply to buy it. The mesh tape made for ducts has two important differences. It's treated to reduce smoke development, and it's cut 3 to 4 in. wide. You can purchase mesh tape where you buy the mastic. Working together, mastic and mesh forms as strong a seal as you can get on ducts.

Characteristics
Unfortunately, there are no uniform standards to help you to judge the sealing qualities of a mastic. You'll have to try several and choose one that works best for you. A good duct mastic has these characteristics:

High solids content - Solids content listed on the product literature should be at least 50 percent. Some have as much as 70 percent. Generally, higher solids means less shrinkage as the material cures.

Excellent adhesion - You'll want the mastic to stick to metal, wood, drywall, plastic, concrete and just about any other material you might find in a house. Since these surfaces are seldom clean, look for a mastic that holds well to dirty or oily surfaces.

Excellent cohesion - The mastic must also stick to itself, so it doesn't crack as the surface moves or "blow out" under the air pressure inside the duct.

Water resistant - Condensation may collect on ducts during cooling, so the mastic must hold even when exposed to water.

Non-toxic - Check the Material Safety Data Sheet for warnings. If you like to spread the material with your hands you may need to wear gloves. Water-based mastics are generally less irritating than pertroleum-based ones.

Surface burning characteristics - Duct mastics should be tested for flame spread and smoke developed (UL723 and ASTME-84). National Fire Protection Association Standard 90A requires duct mastics to have a flame spread rating no higher than 25 and a maximum smoke developed rating of 50.

Viscosity - Some installers like thicker mastic that they can apply with trowel or gloved hand. Others like it a bit thinner, so it brushes on easily. Viscosity is measured in units called centipois (cps). Higher numbers mean thicker mastic. Viscosity around 100,000 cps indicates a consistency similar to mashed potatoes, while 60,000 - 75,000 cps suggests something closer to yogurt.

Storage - Many manufacturers emphasize that their products have a limited shelf life, usually about one year. Most mastics should not be allowed to freeze during storage or shipping.

Color - If ducts will be visible, you may want to select a pleasing color.

Not all these characteristics are published by the manufacturer. Until some type of industry standard is established, you'll just have to try different products.

Application
In most situations, it's easier and more effective to seal ducts with mastic than tape. Even so, to make mastic work, you must install it properly. Here are a few guidelines:

Clean the surface - Wipe loose dirt and oil off the surface with a dry rag. You want the mastic to stick to the duct not the dirt.

Secure the joint - All duct connections must be secured with screws, rivets or another mechanical fastener. Use plastic straps and tightening tools to attach flex ducts to metal collars.

Apply the mastic - Seams, gaps and openings up to 1/4 in. can be sealed with mastic alone. Spread the material at least one inch beyond the opening. Apply a thick coating that fills all crevices.

Provide backing - Gaps between 1/4 in. and 1/2 in. will need additional support. Fiberglass mesh tape made for ducts works well. Holes greater than 1/2 in. may need to be repaired with rigid material.

Precautions
Heating and cooling ducts are just one factor that affects air movement through a house. To avoid an air quality problem in new home construction, be sure to install an effective whole-house ventilation system. If combustion appliances are used, choose sealed combustion models.

Where to Seal

Air Handler

Fill all openings for wiring, plumbing and refrigerant lines.

Seal all seams in the air handler and plenums.

Tape around access panels, so they can be opened for service.

Corners formed by folding sheet metal don't require sealing.